Question

Arithmetic Sequence Need Assistance

Answer 1

let the first term is $a$

by the sum of first n term formula

$\frac{20}{2}(2a+19d)=25a\\20a+190d=25a15\\190d=5a\\a=38d$

substitute to sum of first 30 term

$\frac{30}{2}(2a+29d)=\frac{30}{2}(2(38d)+29d)\\=15(76d+29d)\\=15(105d)\\=1575d$